Abstract
A frequency-domain multiple-input multiple-output (FD-MIMO) equalizer employing a momentum-based gradient descent update algorithm is proposed for polarization multiplexing coherent receivers. Its performance in operation with dynamically varying optical channels is investigated and the impact of filter update delays, arising from the latency of the fast Fourier transforms (FFTs) and other digital signal processing (DSP) operations in the feedback loop, is assessed. We show that the proposed momentum-based gradient descent algorithm used to control the equalizer response has significantly greater tolerance to feedback delay than the conventional gradient descent algorithm. We considered a 92 Gbaud dual-polarization 64 QAM receiver, with DSP operating at two samples per symbol, and with the equalizer operating on blocks of 512 and 1024 samples (i.e., 512/1024-point FFT). We found that at an optical signal-to-noise power ratio (OSNR) of 35 dB, the momentum-based gradient descent algorithm can successfully track state-of-polarization (SOP) rotation at frequencies of up to 50 kHz and with filter update delays of up to 14 blocks (39 ns). In comparison, using the conventional gradient descent algorithm in an otherwise identical receiver, the equalizer performance starts to deteriorate at SOP rotation frequencies above 20 kHz.
Highlights
The use of digital signal processing (DSP) in coherent receivers is a powerful technique to mitigate static and time-varying impairments from optical channels, such as chromatic dispersion (CD), polarization mode dispersion (PMD) and state-of-polarization (SOP) rotation, and a variety of linear filters and adaptive equalizers have been proposed [1,2,3,4,5]
We propose a momentum-based frequency-domain multiple-input multipleoutput (FD-MIMO) equalizer for polarization-multiplexing coherent receivers, and carried out simulations of dual-polarization 92 Gbaud 64 QAM systems over 40 km and 80 km standard single-mode fiber (SSMF) links, with state-of-polarization rotation frequencies of up to 60 kHz
The (GMI)/m was obtained over ten runs of the simulation with random rotation and random phase shift of the received signal at each SOP rotation frequency and optical signal-to-noise power ratio (OSNR) value
Summary
The use of digital signal processing (DSP) in coherent receivers is a powerful technique to mitigate static and time-varying impairments from optical channels, such as chromatic dispersion (CD), polarization mode dispersion (PMD) and state-of-polarization (SOP) rotation, and a variety of linear filters and adaptive equalizers have been proposed [1,2,3,4,5]. We propose a momentum-based frequency-domain multiple-input multipleoutput (FD-MIMO) equalizer for polarization-multiplexing coherent receivers, and carried out simulations of dual-polarization 92 Gbaud 64 QAM systems over 40 km and 80 km standard single-mode fiber (SSMF) links, with state-of-polarization rotation frequencies of up to 60 kHz. The FD-MIMO equalizer combines chromatic dispersion compensation (CDC), matched filtering (MF) and adaptive equalization (AQ), so that frequency to time domain conversions can be avoided to minimize the complexity. We show that the momentum-based gradient descent approach is significantly more robust to feedback latency than the conventional gradient update
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